Polymer-based composites comprising carbon nanotubes as a filler method for producing said composites, and associated uses

ABSTRACT

The invention relates to a method for producing carbon nanotubes in a dispersed state. The method comprises a stage whereby polymerization is carried out from at least one so-called monomer of interest, in the presence of a catalytic system. The catalytic system comprises a cocatalyst/catalyst catalytic couple that is supported by a catalyst carrier, which corresponds to said carbon nanotubes. The invention also relates to composite materials obtained by said method, and to a catalytic system for implementing said method. The invention further relates to the use of the inventive method and products in the field of polymers, especially that of nanotechnologies.

FIELD OF THE INVENTION

The present invention relates to the field of materials and moreparticularly to the field of the composite materials defined below asmicrocomposites and nanocomposites.

The present invention relates especially to a process for obtaining acomposite material comprising a matrix of at least one polymer in whichare dispersed carbon nanotubes serving as filler. The present inventionalso relates to said composites thus obtained and to uses thereof in thefield of nanotechnology.

PRIOR ART

Polymer materials were developed at the start of the 20th-century andthey currently occupy an increasingly important place in our daily life.

For that very reason, industrial pressure is currently such that itdemands increasing specialization of applications, and it is thusnecessary to propose more and more efficient materials to satisfy thisneed.

In the case of polymer materials, this demand implies the provision ofsolutions to overcome the inherent weaknesses of these materials, whichare especially their relative lack of mechanical strength and theirflammable nature.

It has thus been proposed to combine these polymer materials with othercomponents known as “fillers” in order to produce materials known as“composite materials with a polymer matrix” whose properties arereinforced compared with the polymer matrix alone: greater rigidity,better fire resistance, etc.

These fillers may be of fibrillar type such as glass, carbon or Kevlarfibres. These fillers may also be of particulate type such as carbonblacks, silicas, aluminas, calcium carbonates, clays or glass beads.

By way of example, it has been proposed to make copolyolefin-basedcomposites by polymerization of said olefins on fillers in the presenceof a cocatalyst/catalyst couple according to the “Polymerization-FillingTechnique” or “PFT” (Alexandre M. et al. Macromol. Rapid. Comm. (2000),vol. 21, No. 13, pp. 931-936). The catalyst tested was a metallocene,more specificallytert-butylamidodimethyl(tetramethyl-n5-cyclopenta-dienyl)silanedimethyltitanium(CGC) and the cocatalyst was methylaluminoxane (MAO). Various fillerswere tested, including kaolin and graphite. These fillers were of verydifferent nature in terms of composition (inorganic, organic ormetallic) and in terms of morphology and surface properties (acidic orbasic), but they all had in common a specific surface area that wascompatible with the amount of catalyst used, which was relatively small,so as to allow a sufficiently homogeneous deposition of the catalyst atthe surface of these fillers and thus to obtain good polymerizationresults.

Among the composites with a polymer matrix and with particulate fillers,which may be distinguished according to the size of the particles, aremicrocomposites, in which the size of the filler is greater than orequal to one micrometre, and nanocomposites, for which one of the threedimensions of the filler is of the order of one to a few tens ofnanometres.

Nanocomposites have given rise very recently to considerable researchdevelopment. The reason for this is that they are characterized bynoteworthy properties for relatively low filler contents: they result ina substantial improvement in the mechanical properties of the polymermatrix such as the rigidity, and develop a flame-retardant power thatmakes them very advantageous. Furthermore, contrary to fillers offibrillar type, they strengthen the polymer matrix in all directions ofspace [1,2].

More particularly, nanocomposites comprising carbon nanotubes asparticulate filler have already been proposed for various applications[3-6]. Carbon nanotubes are in fact one of the allotropic forms ofcarbon, which may be seen as one or more leaflets of graphite rolledinto a cylinder and sealed at the ends. These carbon nanotubes are,inter alia, characterized by good mechanical properties since theirtensile strength is 40 times greater than that of carbon fibres, andalso good electrical properties, to the point that they have beenproposed for the manufacture of semiconductors or metallic conductors,depending on the structure of the nanotube.

However, in practice, the use of nanotubes as fillers in the polymermatrices for the manufacture of nanocomposites does not appear for thetime being to be able to meet the industrial expectations. Specifically,it turns out that the advantageous properties of carbon nanotubes arenot always transferred to the nanocomposite.

These data are explained by the natural aptitude of carbon nanotubes toaggregate together in very stable packets or “bundles”.

A person skilled in the art is thus confronted with this problem ofaggregation of nanotubes, which limits their use in nanocomposites, andat the present time those skilled in the art are unfortunately stillawaiting an effective solution to overcome this problem.

It will be noted that the use of carbon nanotubes in catalytic systemsis known per se, as attested by documents US-A1-2003/0 119 920 andPatent Abstracts of Japan vol. 2000, No. 6 (20 Sep. 2000).

The first of these documents describes a catalytic system comprising asupport covered with a layer of carbon nanotubes and a catalystcomposition capable of catalysing a chemical reaction. This catalyticsystem may be used in many chemical reactions and especially inpolymerizations. However, the carbon nanotubes are not presented in saiddocument as forming an integral part of the catalytic support itself.

The second of these documents describes a process for depositing carbonnanotubes onto catalytic molybdenum on an inorganic support. In saiddocument, the catalytic system thus comprises (i) carbon nanotubes, (ii)a catalyst that is molybdenum, and (iii) an inorganic support that maybe likened to a cocatalyst, the carbon nanotubes constituting theproduct of the reaction catalysed by said catalyst.

AIMS OF THE INVENTION

The present invention aims to provide a solution that does not have thedrawbacks of the prior art as described above.

In particular, the present invention aims to provide a process forobtaining carbon nanotubes in dispersed form in a polymer matrix, whichmay be used as filler in polymer-based composites and especiallynanocomposites.

The present invention also aims to provide composites, and especiallynanocomposites, comprising a matrix of at least one polymer and at leastcarbon nanotubes acting as fillers, wherein the dispersion of the carbonnanotubes is such that the composite, and especially the nanocomposite,advantageously combines the industrially advantageous physical andchemical properties of said polymer and of said carbon nanotubes.

Another aim of the invention is to provide a process for dispersingcarbon nanotubes in a polymer matrix, the implementation of which isrelatively simple and reasonable in terms of cost compared with theprocesses proposed in the prior art.

SUMMARY OF THE INVENTION

The present invention relates to a process for obtaining carbonnanotubes in dispersed form, characterized in that it comprises a stepof polymerization from at least one monomer, referred to as “monomer ofinterest”, and in the presence of a catalytic system, said catalyticsystem comprising a cocatalyst/catalyst catalytic couple that issupported on a catalysis support, said catalysis support correspondingto said carbon nanotubes.

More specifically, the present invention relates to a process forobtaining a composite material comprising at least one polymer matrix inwhich carbon nanotubes serving as filler are homogeneously dispersed,said process being characterized in that, starting with said carbonnanotubes and a monomer, said carbon nanotubes are used as catalysissupport to bind homogeneously onto the surface thereof acocatalyst/catalyst couple and thus to form a catalytic system, saidcatalytic system is rendered active for polymerization, andpolymerization of said monomer at the surface of the carbon nanotubes isperformed using said active catalytic system, the polymerization beingallowed to progress over time so as thus gradually to obtain, as thepolymerization of the monomer proceeds, the polymer matrix around thecarbon nanotubes, and the composite formed is then recovered.

It is meant by “dispersion of nanotubes in the polymer matrix” means adispersion of nanotubes in said matrix such that the surface area ofcontact between two carbon nanotubes is less than 20% of the totalsurface area of said nanotubes, preferably less than 10%, preferablyless than 5%, less than 2% and preferably less than 1% of the totalsurface area of said nanotubes.

The term “homogeneous” means a distribution of the carbon nanotubes inthe polymer matrix that is homogeneous at least at the microscopicscale, and preferably at the nanoscopic scale.

Preferably, the process according to the invention comprises thefollowing steps:

-   -   preparing a suspension of carbon nanotubes in an inert solvent        is prepared;    -   pretreating said carbon nanotubes by adding said cocatalyst, so        as to obtain a suspension of pretreated carbon nanotubes in        which the cocatalyst is adsorbed onto the surface of the carbon        nanotubes;    -   preparing a reaction mixture from the suspension of carbon        nanotubes thus pretreated, by adding a catalyst and circulating        a flow of monomer in said suspension of pretreated nanotubes so        as to bring about in said reaction mixture the polymerization of        said monomer at the surface of said nanotubes and thus to form a        composite material comprising said polymer of interest and said        carbon nanotubes, in which said carbon nanotubes are coated with        said polymer of interest;    -   stopping the polymerization reaction when the polymerization in        the reaction mixture has reached the desired rate of        polymerization, of between about 0.1% and about 99.9%, and said        composite material thus synthesized is recovered.

Preferably, the monomer of interest is an olefin and the polymer ofinterest is a polyolefin.

Said polyolefin may especially be a hydrophobic polyolefin.

Preferably, said monomer of interest is selected from the groupconsisting of ethylene, propylene, copolymers thereof withalpha-olefins, conjugated alpha-diolefins, styrene, cycloalkenes,norbornene, norbornadiene and cyclopentadiene, and mixtures thereof.

Examples of alpha-olefins include 1-hexene and 1-octene.

Preferably, the polymer of interest is selected from the groupconsisting of ethylene-based polyolefins and propylene-based polyolefinsand mixtures thereof.

Advantageously, the polymer of interest is polyethylene.

Advantageously, in the process of the invention, the cocatalyst/catalystcouple and the experimental parameters are chosen so as to be able toimmobilize the catalyst at the surface of the carbon nanotubes by meansof the cocatalyst and thus to form the catalytic system.

Thus, preferably, the catalyst is chosen such that it is capable ofcatalysing the polymerization of the monomer of interest, said catalystbeing selected from the group consisting of metallocenes, hinderedamidoaryl chelates, hindered oxoaryl chelates, Fe (II and III) and Co(II) bis(imino)pyridines, and Brookhart complexes based on Ni (II) andPd (II), and mixtures thereof.

In general, all these catalysts have in common the fact that they aresoluble complexes of group IV of the chemical elements (Ti, Zr, Hf) thatare active in polymerization.

It will be noted that the metallocenes may be bridged or non-bridged.

Advantageously also, the cocatalyst is methylaluminoxane or a chemicallymodified methylaluminoxane or a mixture thereof.

It is meant by “chemically modified methylaluminoxane” amethylaluminoxane in which about one third of the alkyls are in the formof isobutyls, the rest of said alkyls being in the form of methyls.

In a particularly advantageous manner, the cocatalyst/catalyst catalyticcouple is the methylaluminoxane/Cp*₂ZrCl₂ couple.

Preferably, the amount of catalyst in the process of the invention isbetween about 10⁻⁶ and about 10⁻⁵ mol/g of carbon nanotubes.

Preferably, the amount of cocatalyst in the reaction mixture is betweenabout 10⁻³ and about 10⁻² mol/g of carbon nanotubes.

Advantageously, the temperature of the reaction mixture is between 25°and 140° C.

Preferably, according to the invention, the pretreatment is performed ata temperature of between 25° C. and 200° C. for a time period of between1 minute and 2 hours.

Advantageously, the polymerization is performed at a pressure of betweenabout 1 and about 3 bars of monomer and preferably between 1.1 and 2.7bars of monomer.

Preferably, in order to prepare the reaction mixture, the catalyst isadded to the suspension of pretreated carbon nanotubes beforecirculating the flow of monomer in said suspension.

Alternatively, in order to prepare the reaction mixture, the addition ofthe catalyst to the suspension of pretreated carbon nanotubes and thecirculation of the flow of monomer in said suspension are concomitant.

Preferably, the carbon nanotubes are selected from the group consistingof single-walled carbon nanotubes, double-walled carbon nanotubes andmulti-walled carbon nanotubes, and mixtures thereof.

Preferably, the carbon nanotubes are crude and/or purified carbonnanotubes.

In the process of the invention, the carbon nanotubes may befunctionalized carbon nanotubes.

Preferably, the polymerization reaction is stopped when the rate ofpolymerization is such that the composite comprises between about 50%and about 99.9% of carbon nanotubes and between about 50% and 0.1% ofpolymer.

In the latter case, the process according to the invention preferablyincludes an additional step during which the composite material, oncerecovered, is used as a masterbatch to prepare a nanocomposite based ona polymer referred to as an addition polymer, said addition polymerbeing miscible and compatible with the polymer of interest of thecomposite material.

Alternatively, the polymerization reaction is stopped when the rate ofpolymerization is sufficient to obtain in sufficient amount a compositematerial corresponding to a nanocomposite and comprising a matrix ofsaid polymer of interest in which the carbon nanotubes are homogeneouslydispersed at the nanoscopic scale.

More specifically, the polymerization reaction is preferably stoppedwhen the nanocomposite formed comprises between about 0.1% and about 50%of carbon nanotubes and between about 99.9% and about 50% of polymer.

The present invention also relates to a catalytic system for performingthe process described above and comprising at least carbon nanotubes, acocatalyst and a catalyst, said catalyst forming with said cocatalyst acatalytic couple, in which said catalyst and said cocatalyst are boundto the surface of said carbon nanotubes.

The present invention also relates to a composition for performing thisprocess and comprising said catalytic system.

The present invention also relates to a composite material obtained bythe process described above.

This composite material comprises between about 0.1% and 99.9% of carbonnanotubes and between about 99.9% and 0.1% of polymer.

According to a first preferred embodiment of the invention, thecomposite material corresponds to a nanocomposite and comprises at leastone matrix of at least one polymer of interest in which carbon nanotubesare homogeneously dispersed at the nanoscopic scale in the form offillers.

Preferably, this composite material comprises between about 0.1% andabout 50% of carbon nanotubes and between about 99.9% and about 50% ofpolymer.

Preferably, in the composite material according to the invention, thecarbon nanotubes are covered or coated with the polymer.

The invention also relates to a composite material comprising a matrixof at least one addition-polymer and the composite material as describedabove.

Another subject of the invention is the use of the process, and/or ofthe catalytic system, and/or of the composition, and/or of the compositematerial described above in applications in the field of nanotechnology.

The present invention also relates to a process for polymerizing amonomer referred to as a monomer of interest, characterized in that ituses the process according to the invention, the polymerization reactionbeing allowed to proceed for long enough to have a proportion of carbonnanotubes of less than 0.1% and a proportion of polymer of greater than99.9%.

The invention also relates to a polymer obtained by this process.

Definitions

Reference will be made to the preceding paragraphs to understand what ismeant in the present invention by the terms “nanocomposites”.“microcomposites” “aggregation/deaggregation”, “dispersion”, “polymermatrix” and “filler”.

It will be noted that the polymer matrix comprises at least one polymer.

The term “composite materials” relates to both microcomposites andnanocomposites.

To understand specifically what is meant by the term “compositematerial” or “composite” in the present invention, reference may be madeto the document “Materiaux et composites [Materials and composites]”from Berthelot J. M., published by Tec & Doc, 3rd edition, Paris 1999,p. 3.

The term “catalyst” has in the present invention the same meaning asthat used by a person skilled in the art in the field of chemistry. Thisterm denotes a compound which, when used at very low concentration in areaction medium, allows the rate of a polymerization reaction to beincreased via interaction with the reagents, without, however, beingchemically altered at the end of the reaction.

The term “cocatalyst” has in the present invention the same meaning asthat used by a person skilled in the art in the field of chemistry. Thisterm denotes a compound capable of acting synergistically with thecatalyst to increase the rate of the polymerization reaction.

The term “poison” means a compound that inhibits a polymerizationreaction.

The carbon nanotubes are referred to as being “single-walled”,“double-walled” or “multi-walled” depending on whether the nanotubesconsist, respectively, of one, two or several leaflets as describedabove. This terminology is well known to those skilled in the art.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a schematically shows the principle of the process according tothe present invention applied to the case of polyethylene.

FIG. 1 b shows the activation of the zirconocene catalyst at the surfaceof the nanotubes according to the process of the invention.

FIG. 2 presents a comparison of the polymerization kinetics curves forthe polymerization of ethylene in the presence and in the absence ofcrude multi-walled carbon nanotubes (MWNTs).

FIG. 3 presents a comparison of the kinetics curves for thepolymerization of ethylene according to the process of the inventionusing a catalytic system comprising or not comprising crude multi-walledcarbon nanotubes.

FIG. 4 a corresponds to an SEM electron microphotograph taken on asample of crude MWNT carbon nanotubes.

FIG. 4 b corresponds to a zoom taken on the sample of FIG. 4 a tovisualize the bundles or aggregates.

FIG. 4 c corresponds to an SEM electron microphotograph taken on asample containing crude MWNT nanotubes with 10% by weight ofpolyethylene and obtained with binding of MAO cocatalyst according tothe process of the invention.

FIG. 4 d corresponds to a zoom taken on the sample of FIG. 4 c.

FIG. 4 e corresponds to an SEM electron microphotograph taken on asample containing crude MWNT nanotubes with 42% by weight ofpolyethylene and obtained with binding of MAO cocatalyst according tothe process of the invention.

FIG. 4 f corresponds to a zoom taken on the sample of FIG. 4 e.

FIG. 5 a is a microphotograph taken by transmission electron microscopy(TEM) of the sample photographed in FIGS. 4 a and 4 b and correspondingto crude MWNT carbon nanotubes alone.

FIG. 5 b is a microphotograph taken by transmission electron microscopy(TEM) of the sample photographed in FIGS. 4 e and 4 f and correspondingto crude MWNT nanotubes with 42% by weight of polyethylene.

FIG. 5 c is a microphotograph taken by transmission electron microscopy(TEM) of a sample corresponding to crude MWNT nanotubes with 74% byweight of polyethylene and obtained with binding of MAO cocatalystaccording to the process of the invention.

FIG. 6 gives a comparison of the TGA thermograms in air for differentmixtures based on HDPE matrix. Sample Dabo40 a corresponds to a mixturecontaining the HDPE matrix alone, the sample Dabo40 b corresponds to asimple mixture of an HDPE matrix and of crude MWNT carbon nanotubes (2%by weight), and the sample Dabo40 c corresponds to a mixture of an HDPEmatrix and of a masterbatch containing crude MWNT carbon nanotubes (2%by weight) as obtain according to the process of the invention.

FIG. 7 is a photograph taken after combustion of the sample Dabo40 b asdefined in the preceding paragraph.

FIG. 8 is a photograph taken after combustion of the sample Dabo40 c asdefined in this same paragraph.

FIG. 9 a is a TEM microphotograph of the same sample Dabo40 b and FIG. 9b corresponds to a zoom on the zone containing a bundle of MWNT carbonnanotubes.

FIG. 10 a is a TEM microphotograph of the same sample Dabo40 c and FIG.10 b corresponds to a zoom showing an isolated MWNT carbon nanotube.

FIG. 11 compares the polymerization kinetics for a simple polymerizationof ethylene and for a polymerization of ethylene performed onsingle-walled carbon nanotubes (SWNTs) according to the process of theinvention.

FIGS. 12 to 16 compare the results of tensile tests obtained for threetypes of sample: high-density polyethylene alone; a composite obtainedby simple mixing of high-density polyethylene with 1% of multi-walledcarbon nanotubes (MWNTs); a composite obtained from high-densitypolyethylene and a masterbatch prepared according to the invention.

More specifically, FIGS. 12 to 16 compare the following parameters:

FIG. 12: breaking stress

FIG. 13: elongation at break

FIG. 14: Young's modulus

FIG. 15: yield-point stress

FIG. 16: elongation at the yield point

DETAILED DESCRIPTION OF THE INVENTION

The novelty of the present invention resides in the idea of proceeding,to prepare polyethylene/carbon nanotube nanocomposites, via a step ofpolymerization on a treated filler, according to the technique known asthe “Polymerization-Filling Technique” and abbreviated to “PFT” [8].

During this polymerization step, a catalyst known for catalysing thepolymerization of the monomer of interest is bound to the surface of thefiller, i.e. in this case pretreated carbon nanotubes advantageously insuspension form. The polymerization of the monomer under considerationis then initiated directly on the surface of this same filler.

The pressure at the surface of the nanotubes, brought about by thepolymerization during this step, allows, in an entirely unexpectedmanner, according to the invention, the deaggregation of the bundlesthat are usually associated with the formation of nanocompositescomprising carbon nanotubes. A dispersion of carbon nanotubes in theform of filler in the polymer matrix is thus obtained in thenanocomposite, and this dispersion is homogeneous at the nanoscopicscale.

It will be noted that the pretreatment of the carbon nanotubes consistsin binding to the surface of the carbon nanotubes a cocatalyst selectedon the basis of being catalytically active in cooperation with thecatalyst during the polymerization of the monomer. In other words, thismeans that the cocatalyst and the catalyst are chosen to form together acatalytic couple for the polymerization of the monomer and that thecarbon nanotubes may be viewed as being a catalysis support that defineswith said catalytic couple a catalytic system.

It should be understood that, according to the present invention, thebinding of the catalyst to the surface of the carbon nanotubes isperformed via the cocatalyst, such that the catalyst and the cocatalystare chemisorbed onto the surface of the nanotubes.

The present invention is illustrated by means of the particular exampleof ethylene and of the preparation of nanocomposites based on apolyethylene matrix.

However, the invention also relates to other polymers and othernanocomposites that a person skilled in the art may easily identify onthe basis of the present description, as underlined above.

This is likewise the case for the nature of the catalyst and that of thecocatalyst.

It will be noted, however, that a necessary condition for performing theprocess according to the invention is that the polymer of interestformed is insoluble in the polymerization medium (reaction mixture) suchthat it can precipitate at the very site of its polymerization, i.e. onthe surface of the nanotubes.

FIG. 1 a summarizes the principle of the process according to thepresent invention applied to the case of polyethylene. As shown in thisfigure, ethylene is polymerized directly at the surface of the nanotubesin suspension, which leads to gradual coating or covering of the carbonnanotubes with polyethylene as it is synthesized. The effect of thiscoating is to force the carbon nanotubes to become separated from eachother and thus to bring about deaggregation of the bundles of nanotubes.

The cocatalyst used by way of example and with which the carbonnanotubes are pretreated is methylaluminoxane, abbreviated to MAO. Thiscocatalyst is used in the form of a solution.

The cocatalyst is such that it is capable of interacting viainteractions of Lewis acid-base type with the wall of the carbonnanotubes, which is rich in π electrons.

The catalyst used by way of example,bis(1,2,3,4,5-pentamethylcyclopentadienyl)zirconium (IV) dichloride orCp*₂ZrCl₂, is then added. It is also in the form of a solution.

It will be noted that, on contact with the MAO-treated filler, CP*₂ZrCl₂gives rise to a cationic species that is active in the polymerization ofethylene and to a negative counterion located on the treated filler, asillustrated by FIG. 1 b.

By means of this binding of the active species, the ethylene polymerizesat the surface of the filler. Since polyethylene is insoluble in thesolvents used, it becomes deposited by precipitation directly onto thesurface of the filler (i.e. the carbon nanotubes).

Reagents Used:

The fillers used were multi-walled carbon nanotubes (MWNTs). Two typesof nanotube were used, namely crude MWNTs still containing 30% by weightof catalytic impurities (mainly about 30% by weight of alumina, 0.3% byweight of cobalt and 0.3% by weight of iron) and purified MWNTscontaining traces of catalytic impurities (0.2% by weight of alumina,0.3% by weight of iron and 0.3% by weight of cobalt). These nanotubeswere provided by the department of Professor J. B. Nagy of the FacultésUniversitaires Notre Dame de la paix, Namur.

The solvent used was n-heptane (analytical grade, from the companyAcros). It was dried over molecular sieves of porosity equal to 0.4nanometre (also supplied by the company Acros).

Certain solutions were prepared using dry toluene. To do this, thetoluene (analytical grade) supplied by the company Labscan was dried byboiling it over calcium hydride, and was then freshly distilled.

The selected cocatalyst was methylaluminoxane (MAO) from the companyAtofina. Solutions at between 3M and 0.2M of MAO in toluene were usedfor the syntheses.

Bis(1,2,3,4,5-pentamethylcyclopentadienyl)zirconium (IV) dichloride(CP*₂ZrCl₂) (from the company Aldrich) was used herein as catalyst. Itwas dissolved in dry toluene so as to form solutions of between 1 and 10mM.

The ethylene (99.998%) (from the company Air Liquide) was used assupplied, in gas form.

Methanol (technical grade) (from the company Brenntag) was used toprecipitate and to recover the polymer or the composite.

Pretreatment of the Carbon Nanotubes with the Cocatalyst:

The crude nanotubes (between 0.25 g and 1 g) were introduced into a 250ml or 500 ml two-necked round-bottomed flask (according to thesubsequent use) containing a magnetic stirring bar and equipped with aglass three-way tap (stoppered with a rubber septum).

The flask, connected to a vacuum trap immersed in liquid nitrogen inorder to recover the adsorbed water, was conditioned, i.e. flame-driedunder vacuum using a. Bunsen burner. The nanotubes were then driedovernight at between 100 and 150° C. under vacuum and with magneticstirring. The flask was placed under a slight positive pressure ofnitrogen.

100 ml of n-heptane were then introduced into the flask. An amount ofMAO (between 0.001 and 0.01 mol/g of MWNTs) was freed beforehand of itstrimethylaluminium by evaporation to dryness under vacuum. Theevaporated trimethylaluminium was condensed in a vacuum trap immersed inliquid nitrogen and stored for analysis. This removal of thetrimethylaluminium (TMA) is necessary since it has been demonstratedthat TMA, which is an excellent Lewis acid, is an effective competitorof MAO as regards the adsorption [7].

The solid MAO was redissolved in toluene and transferred by capillary,under nitrogen, onto the suspension of MWNTs in heptane.

The system was thermostatically maintained at between 20 and 60° C.using an oil bath with magnetic stirring for between 10 minutes and 2hours.

The MAO was concentrated to dryness under vacuum in the presence of theMWNTs, and the solvents removed were condensed in a round-bottomed flaskimmersed in liquid nitrogen (filler treatment fraction).

The bath temperature was then raised to high temperature (between 100and 200° C.) for between 30 minutes and 3 hours, while leaving the flaskunder vacuum (10⁻¹ torr) to bind the MAO to the carbon nanotubes. Theflask was then placed again under a slight positive pressure ofnitrogen.

Washing was then performed three times in order to remove the MAO notbound to the filler. To do this, 80 ml of dry toluene were added to thenanotubes and stirred for 5 minutes at 60° C. The MAO-treated filler wasallowed to settle without stirring. The supernatant was then removedusing a conditioned capillary and under a flow of nitrogen. The threesolutions and the filler treatment fraction (n-heptane) were thenpooled.

The small amount of residual toluene present in the flask was entrainedunder vacuum and concentrated in a flask immersed in liquid nitrogen soas to add it thereafter to the filler treatment fraction.

After this treatment, the flask thus contained pretreated nanotubes.

Homopolymerization of Ethylene in the Presence of Carbon NanotubesPretreated with the Cocatalyst

It will be noted that the binding of the catalyst to the pretreatednanotubes was performed in inert medium, maintained under a slightpositive pressure of nitrogen, while avoiding the presence of proticimpurities and of oxygen in the reaction medium.

To perform the homopolymerization of ethylene, 100 ml of n-heptane wereadded to the flask containing the MAO-treated nanotubes.

The mixture was then transferred under a nitrogen atmosphere into apreconditioned round-bottomed reactor containing a magnetic stirringbar.

Between 1×10⁻⁵ and 1×10⁻⁶ mol of Cp*₂ZrCl₂ per g of nanotubes was added,under a nitrogen atmosphere. The round-bottomed reactor was thenthermostatically maintained in an oil bath at between 25° and 100° C.(polymerization temperature) for between 5 and 60 minutes.

The medium was then purged for 30 seconds with a flow of ethylene. Thepolymerization was performed for one hour at a pressure of between 1.1and 2.7 bars of ethylene. The composite thus synthesized was thenrecovered by precipitating it from methanol acidified with 12Mhydrochloric acid.

Description of a First Preferred Embodiment of the Invention

1. Reagents Used:

The fillers used were multi-walled carbon nanotubes (MWNTs). Two typesof nanotubes were used, i.e. crude MWNTs still containing 30% by weightof catalytic impurities (mainly about 30% by weight of alumina, 0.3% byweight of cobalt and 0.3% by weight of iron) and purified MWNTscontaining traces of catalytic impurities (0.2% by weight of alumina,0.3% by weight of iron and 0.3% by weight of cobalt). These nanotubeswere provided by the department of Professor J. B. Nagy of the FacultésUniversitaires Notre Dame de la Paix, Namur.

The solvent used was n-heptane (analytical grade, from the companyAcros). It was dried over molecular sieves of porosity equal to 0.4nanometres (also supplied by the company Acros).

Certain solutions were prepared using dry toluene. To do this, thetoluene (analytical grade) supplied by the company Labscan was dried byboiling it over calcium hydride, and was then freshly distilled.

The selected cocatalyst was methylaluminoxane (MAO) from the companyAtofina. A 1.45M solution of MAO in toluene was used for the synthesis.

Bis(1,2,3,4,5-pentamethylcyclopentadienyl)zirconium (IV) dichloride(Cp*₂ZrCl₂) (from the company Aldrich) was used herein as catalyst. Itwas dissolved in dry toluene so as to form a solution of about 5 mM.

The ethylene (99.998%) (from the company Air Liquide) was used assupplied, in gas form.

Methanol (technical grade) (from the company Brenntag) was used toprecipitate and to recover the polymer or the composite.

2. Pretreatment of the Carbon Nanotubes with the Cocatalyst:

The crude nanotubes (0.25 g or 1 g depending on the case) wereintroduced into a 250 ml or 500 ml two-necked round-bottomed flask(according to the subsequent use) containing a magnetic stirring bar andequipped with a glass three-way tap (stoppered with a rubber septum).

The flask, connected to a vacuum trap immersed in liquid nitrogen inorder to recover the adsorbed water, was conditioned, i.e. flame-driedunder vacuum using a Bunsen burner. The nanotubes were then driedovernight at 100° C. under vacuum and with magnetic stirring. The flaskwas placed under a slight positive pressure of nitrogen.

100 ml of n-heptane were then introduced into the flask. An amount ofMAO with an Al concentration of 1.45M (32 ml/g of MWNTs or 46 mmol/gramof MWNTs) was freed beforehand of its trimethylaluminium by evaporationto dryness under vacuum. The evaporated trimethylaluminium was condensedin a vacuum trap immersed in liquid nitrogen and stored for analysis.This removal of the trimethylaluminium (TMA) is necessary since it hasbeen demonstrated that TMA, which is an excellent Lewis acid, is aneffective competitor of MAO as regards the adsorption [7].

The solid MAO was redissolved in toluene and transferred by capillary,under nitrogen, onto the suspension of MWNTs in heptane.

The system was thermostatically maintained at 40° C. in an oil bath withmagnetic stirring for one hour.

The MAO was concentrated to dryness under vacuum in the presence of theMWNTs, and the solvents removed were condensed in a round-bottomed flaskimmersed in liquid nitrogen (filler treatment fraction).

The bath temperature was then raised to high temperature (150° C.) forone and a half hours, while leaving the flask under vacuum (10⁻¹ torr)to bind the MAO to the carbon nanotubes. The flask was then placed againunder a slight positive pressure of nitrogen.

Washing was then performed three times in order to remove the MAO notbound to the filler. To do this, 80 ml of dry toluene were added to thenanotubes and stirred for 5 minutes at 60° C. The MAO-treated filler wasallowed to settle without stirring. The supernatant was then removedusing a conditioned capillary and under a flow of nitrogen. The threesolutions and the filler treatment fraction (n-heptane) were thenpooled.

The small amount of residual toluene present in the flask was entrainedunder vacuum and concentrated in a flask immersed in liquid nitrogen soas to add it thereafter to the filler treatment fraction.

After this treatment, the flask thus contained pretreated nanotubes.

3. Homopolymerization of Ethylene in the Presence of Carbon NanotubePretreated with the Cocatalyst:

It will be noted that the binding of the catalyst to the pretreatedcarbon nanotubes was performed in inert medium, maintained under aslight positive pressure of nitrogen, while avoiding the presence ofprotic impurities and of oxygen in the reaction medium.

To perform the homopolymerization of ethylene, 100 ml of n-heptane wereadded to the flask containing the MAO-treated nanotubes (0.25 g).

The mixture was then transferred into a preconditioned round-bottomedreactor containing a magnetic stirring bar.

2.2 ml of the solution of 5 mM Cp*₂ZrCl₂ (11.5 μmol for the 0.25 g ofnanotubes) [lacuna]. The round-bottomed reactor was thenthermostatically maintained in an oil bath at 50° C. (polymerizationtemperature) for 15 minutes.

The medium was then purged for 30 seconds with a flow of ethylene. Thepolymerization was performed for one hour at a pressure of 2.7 bars ofethylene. The composite thus synthesized was then recovered byprecipitating it from methanol acidified with 12M hydrochloric acid.

4. Results

4.1. Absence of Catalytic Activity Associated with the Carbon NanotubesAlone:

It will be noted that, for the needs of the test, the polymerizationtime period was reduced from 60 to 30 minutes.

It was demonstrated that the crude carbon nanotubes showed no catalyticactivity in the polymerization of ethylene.

To do this, polymerization reactions were performed under variousconditions corresponding to various samples, and the results obtainedare presented in Table 1.

As shown by this Table 1, when the polymerization is performed in theabsence of the MAO cocatalyst (see samples 24 b and 28 a), no polymer isobtained, whereas a small amount of polyethylene was obtained in thepresence of MAO (see sample Dabo24 a).

Verification was made that the product obtained in the presence of MAOwas indeed polyethylene (data not presented herein).

4.2. Absence of Poison Effect on the Carbon Nanotubes with Respect tothe Catalytic Couple:

Tests, the results of which are presented in Table 2 and in FIG. 2, madeit possible to demonstrate that the crude multi-walled nanotubes did notconstitute a poison for the MAO/Cp*₂ZrCl₂ catalytic couple.

Specifically, if the consumption of ethylene over time for apolymerization performed in the absence and in the presence ofnon-pretreated crude multi-walled carbon nanotubes is compared, it isobserved that the catalytic activity is identical in both cases (seeTable 2) and that the ethylene consumption curves are virtuallysuperimposed in both cases (see FIG. 2).

It will be noted that the catalytic activity is defined as the amount inkg of polyethylene (PE) produced per mole of Zr and per hour.

4.3. Use of Carbon Nanotubes as Catalysis Support in the Polymerizationof Ethylene:

It has been demonstrated according to the present invention, entirelysurprisingly, that the carbon nanotubes can be used as catalysis supportfor the MAO/Cp*₂ZrCl₂ couple in the polymerization of ethylene.

This result is all the more surprising since a person skilled in theart, knowing the abundance of π electrons on the carbon nanotubes, whichmake them efficient Lewis bases and thus theoretically capable ofcompeting with olefinic monomers, for instance ethylene, would on thecontrary have expected deactivation or even inhibition of thepolymerization.

To do this, polymerization tests in the presence and absence ofpretreated crude multi-walled carbon nanotubes were performed, and theresults of these tests are presented in Table 3 and in FIG. 3.

As shown by Table 3, a 50% increase in the catalytic activity isobserved for the sample Dabo 21, corresponding to a polymerization inthe presence of pretreated nanotubes, compared with that obtained forthe sample Dabo23 (in the absence of said nanotubes). This means thatthe catalytic system formed by the catalytic couple ofcocatalyst/catalyst supported by carbon nanotubes has catalytic activityin the polymerization of ethylene higher than that of the couple alone.

Comparison of the polymerization kinetics as presented in FIG. 3confirms these results.

4.4. Thermal Characterization of the PE/Crude MWNT Composites:

Composites with an increasing content of polyethylene were synthesizedby taking various fractions during polymerization, and the change in themelting point and that of the degree of crystallinity duringpolymerization were measured using these various composites bydifferential scanning calorimetry (DSC) and by thermogravimetricanalysis (TGA).

The corresponding results are presented in Table 4. As these resultsshow, there is an increase in the melting point gradually as theproportion of synthesized polyethylene increases, whereas the degree ofcrystallinity also increases until it reaches a threshold value (66%).

These results demonstrate the formation of composites with an increasingproportion of polyethylene. Furthermore, the thermal properties of thesecompounds increase as the proportion of polyethylene they containincreases.

4.5. Morphological Characterization of the Crude PE/MWNT Composites:

FIGS. 4 a to 4 f show SEM microphotographs obtained for various crudePE/MWNT composites comprising variable weight contents of polyethylenerelative to the carbon nanotubes and ranging from 0% to 42%.

In a sample containing only carbon nanotubes, it is observed, as shownin FIGS. 4 a and 4 b, according to a phenomenon well known to thoseskilled in the art, that these carbon nanotubes have a natural andspontaneous tendency to aggregate in the form of packets, lumps orbundles. These bundles are indicated by an arrow.

Using this sample as a point of comparison, it is found that as theweight content of synthesized polyethylene in the composite increases,these bundles have an increasing tendency to deaggregate (FIGS. 4 c to 4f).

It will be noted that complementary morphological analyses performed byscanning electron microscopy (SEM) (not shown herein) confirmed thistendency towards destructuring of the “bundles” of carbon nanotubes forcontents of from 50% to 75% by weight of polyethylene

In order to visualize the coating of the nanotubes, various samples wereanalysed by transmission electron microscopy (TEM), these samplescontaining 0%, 42% and 74% by weight of polyethylene. The photographsobtained for these various samples are shown in FIGS. 5 a to 5 c.

As may be seen, for the sample corresponding to polyethylene-free crudeMWNT nanotubes, “bundles” of carbon nanotubes are observed (see FIG. 5a). These bundles contain nanotubes of various diameters ranging fromabout 10 to about 40 nanometres. They are several micrometres long. Itwill be noted that the object visible at the centre of the photographmight be likened to a catalyst particle (containing cobalt and iron)used for the production of nanotubes.

On the other hand, for a sample corresponding to crude nanotubescontaining 42% by weight of polyethylene (FIG. 5 b), partial coating ofthe carbon nanotubes with polyethylene is observed on the edges of thesample (see the arrow in FIG. 5 b).

In comparison, as shown in FIG. 5 c, the proportion of crude nanotubescoated (covered) with polyethylene increases for a sample containing agreater proportion of polyethylene (crude nanotubes containing 74% byweight of polyethylene). In this figure, the coated nanotubes areindicated by a black arrow, whereas the areas very rich in polyethyleneare indicated by a dashed arrow. This coating is especially visible onthe edges of the sample.

4.6. Effect of Purification of the Carbon Nanotubes on the PE/MWNTComposite Obtained:

A measurement of the catalytic activities similar to that describedabove for crude carbon nanotubes was performed with purified carbonnanotubes (data not shown).

This study made it possible to demonstrate that the catalytic system inwhich the support consists of purified carbon nanotubes is just asefficient in terms of catalytic activity as that in which the supportconsists of crude carbon nanotubes.

In a similar manner to the study presented above for composites in whichthe multi-walled nanotubes were crude, a study was performed onPE/multi-walled nanotube (MWNT) composites in which the carbon nanotubeshad been purified.

The results obtained (not shown herein) demonstrated that the thermalproperties and the morphological characteristics of thesePE/multi-walled carbon nanotube composites obtained with purified carbonnanotubes were comparable with those obtained with crude carbonnanotubes.

In addition, these results made it possible to show that lesspolyethylene was needed to destructure the “bundles” of carbon nanotubeswhen they are purified than when they are crude. This may be explainedby the fact that the purification of the carbon nanotubes reduces theirstructuring in bundles. Specifically, purification makes it possible toremove the catalytic residues located at the base of the “bundles”,which partly ensure their cohesion.

4.7. Use of PE/MWNT Nanotube Composites as Masterbatches for PreparingComposites:

In order to be able to determine the provision of polymerization ofethylene on the MAO-treated crude multi-walled carbon nanotubesaccording to the present invention, melt blends were prepared.

In order for these blends to be comparable, the matrix needed to beidentical. To do this, an HDPE matrix (from the company Dow) with a meltflow index by mass of 1.1 g/10 minutes under 2.16 kg and 190° was used.

The preparation of these blends with this matrix was performed byblending in a closed internal chamber (Brabender), followed by formingof the material thus obtained by compression in a mould.

More specifically, the polymer matrix and the filler(polyethylene-coated carbon nanotubes) were melted and mixed together inthe closed-chamber blender. Once this material was molten and thoroughlymixed, it was transferred into a suitable stainless-steel mould whosesurface is covered with a Teflon film. The whole was then hot-pressed(to fully take the shape of the mould) and then cold-pressed (to set thematerial) by means of a hydraulic press. Composite plaques 3 mm thickwere thus obtained.

The blender used herein was a Brabender internal blender (˜70 g ofpolymer) and the press was an Agila PE20 hot/cold hydraulic doublepress. The blending was performed at 190° C. (screw speed: 45 rpm) for atime period of 2 minutes to melt and mix the HDPE alone and then for 10minutes to mix it with the filler. The pressing procedure was asfollows: 3 minutes at low pressure and at 190° C., three minutes at 150bars and at 190° C. and finally five minutes without heating, at apressure of 150 bars.

Three mixtures were thus prepared by blending in an internal chamber.These were the HDPE matrix alone (sample Dabo 40 a), the HDPE matrixcontaining 2% by weight of crude MWNTs not pretreated with MAO (sampleDabo 40 b) and the matrix containing 2% by weight of crude MWNTs treatedwith MAO and coated via in situ polymerization of ethylene, used inmasterbatch form (sample Dabo 40 c). This “masterbatch” is in factobtained by combining several samples obtained from the polymerizationof ethylene on the MAO-treated crude multi-walled carbon nanotubesaccording to the process of the invention and from which samples weretaken during polymerization. In this “masterbatch”, the proportion ofpolyethylene generated in situ according to the process of the inventionis about 70% by weight relative to the amount of crude MWNTs.

4.7.1. Mechanical and Viscoelastic Properties

On each of the mixtures, the mechanical properties were determined bymeans of tensile tests and averaged over a minimum of 5 samples. Thetensile speed was 50 mm/minute. Furthermore, the viscoelastic propertieswere also determined using a “Melt Flow Indexer” (MFI).

The results obtained are summarized in Table 5.

The parameters summarized in this table are well known to those skilledin the art. As a reminder, the mechanical parameters of a sample aredefined in this table in the following manner:

-   -   the Young's modulus (E) is the characteristic of the initial        strain strength of the material (rigidity);    -   the “yield point stress” and the “yield point strain”        correspond, respectively, to the stress value and the elongation        value at the flow threshold (σ_(s), ε_(s));    -   the “MFI” characterizes the melt viscosity of the material.

The results of Table 5 show that the addition of nanotubes does notsignificantly influence the rigidity of the HDPE matrix (see thirdcolumn of Table 5, “Young's modulus”).

On the other hand, the addition of these nanotubes is accompanied by adecrease in the breaking strain (second column of Table 5).

However, the use of the “masterbatch” makes it possible to conserveultimate properties such as the relatively high breaking stress andbreaking strain, which are characteristic of maintenance of theductility of the material (comparison between samples Dabo 40 b and Dabo40 c).

As regards the viscoelastic properties (final column of Table 5, “MFI”),the addition of MWNT carbon nanotubes greatly reduces the MFI, i.e. theaddition of the carbon nanotubes tends to increase the melt viscosity ofthe material.

However, by comparison, the pretreatment by polymerization of ethyleneon MAO-treated nanotubes according to the process of the invention leadsto a smaller reduction in the MFI factor, i.e. the increase in the meltviscosity is smaller than with a simple mixture (Dabo 40 b). This may beexplained by the fact that the process according to the invention allowsbetter dispersion of the carbon nanotubes and thus an MFI factor that isrelatively higher compared with the simple mixture. The betterdispersion of the MWNTs and thus the destructuring of the “bundles” arequite probably the cause of this increase in melt viscosity, whichincreases the flow of the matrix through the standardized die.

In conclusion, the pretreatment of carbon nanotubes by polymerization ofethylene on MAO-treated nanotubes according to the invention makes itpossible to improve the mechanical properties of the composite obtainedinsofar as it allows a better compromise between the rigidity and theductility of the material, while at the same time maintaining aviscosity that is suited to the intended use of the material.

4.7.2. Thermal Properties

Thermal analyses, by differential scanning calorimetry (DSC) and bythermogravimetric analysis (TGA), were performed and compared for thevarious mixtures. The data obtained are summarized in Table 6 and inFIG. 6.

These data demonstrate that the melting point is slightly higher for thecomposites than for the matrix alone (first column, “m.p.” of Table 6).

Very interesting data are constituted by the fact that for the compositecontaining the nanotubes pretreated with MAO and coated withpolyethylene according to the process of the invention (sample Dabo 40c), the gain in heat stability is even better than for the simplemixture.

It is also found, very advantageously, that in the presence of crudeMWNTs (Dabo 40 b and Dabo 40 c entries), the degradation temperature inan oxidizing atmosphere (in air) of the HDPE matrix is markedly higher(about 50° C.) compared with that of the HDPE matrix alone (sample Dabo40 a) (“T_(deg.) in air” column). This is clearly demonstrated in thethermograms of FIG. 6.

In other words, whereas the simple mixing of carbon nanotubes with theHDPE matrix makes it possible to improve the stability of the HDPEmatrix, it emerges from the present invention that the presence ofcarbon nanotubes, even in an amount as small as 2% by weight, which haveundergone the treatment according to the process of the invention, makesit possible to even further improve this heat stability.

4.7.3. Fire Behaviour

The test performed consisted in burning a sample and in observing thebehaviour of the material during combustion: possible formation ofignited drops capable of propagating the fire to the surrounding medium,deformation of the material, intensive volatilization, etc.

When the sample corresponding to the HDPE matrix alone (Dabo 40 a) wasignited, it burned generating ignited drops. The propagation of theflame along the sample was rapid, leading to the combustion of all ofthe initial sample.

In comparison, the combustion of the sample corresponding to a compositewith an HDPE matrix containing 2% by weight of crude MWNTs obtained bysimple mixing (Dabo 40 b), there was no longer any formation of igniteddrops. The propagation of the flame was slower than in the case of theHDPE matrix alone. After combustion, the sample, although deformed, asshown by FIG. 7, conserved overall its initial dimensions. Thisobservation is typical of the “charring” phenomenon resulting from thecarbonization of the organic matrix induced by the presence ofnanofillers and resulting in the formation of a carbonized crust(“char”).

The treatment of the carbon nanotubes according to the invention doesnot in principle make it possible to further improve the fire behaviourof the HDPE matrix, as evidenced by FIG. 8 (sample Dabo 40 c), comparedwith carbon nanotubes not treated according to this process (sample Dabo40 b).

Nevertheless, the behaviour of this composite with an HDPE matrixcontaining 2% by weight of crude MWNTs treated according to the processof the invention appears to be better than that of the matrix alone. Asin the case of the composite containing untreated carbon nanotubes, theformation of a crust (“char”) in the total absence of ignited flow isobserved (see FIG. 8).

In conclusion, the incorporation of carbon nanotubes treated accordingto the process of the invention into an HDPE matrix allows the formedcomposite to burn without formation of ignited droplets and with a muchslower propagation speed compared with the HDPE matrix alone. However,in principle, there does not appear to be any significant difference inthe fire behaviour between the composites obtained by simple mechanicalmixing of HDPE and of crude MWNT nanotubes and those obtained by mixingHDPE and MWNTs treated according to the process of the invention.

4.7.4. Morphological Characterizations

So as to visualize the state of deaggregation of the carbon nanotubes,morphological characterizations by transmission electron microscopy(TEM) were performed on the two composites Dabo 40 b and Dabo 40 cobtained, respectively, by mixing HDPE and crude MWNTs, on the one hand,and by mixing HDPE and MWNTs treated according to the process of theinvention, on the other hand. The corresponding photographs arepresented in FIGS. 9 a, 9 b and 10 a, 10 b, respectively.

It is seen that the melt blending of the crude MWNTs with HDPE(composite Dabo 40 b) is not efficient enough to deaggregate the“bundles” of nanotubes. Specifically, in FIGS. 9 a and 9 b of thiscomposite, “bundles” of nanotubes may be seen (see arrows andmagnification).

By comparison, the morphological analysis performed by transmissionelectron microscopy (TEM) on the composite Dabo 40 c containing thecarbon nanotubes treated according to the process of the inventionreveals better dispersion of the carbon nanotubes within the matrix (seeFIGS. 10 a and 10 b), since the “bundles” of nanotubes can no longer beseen. In contrast, carbon nanotubes that are relatively separated fromeach other are seen (see FIG. 10 a). The dispersion of the nanotubesappears to be relatively homogeneous throughout the sample.

In conclusion, the observations made by transmission electron microscopy(TEM) demonstrate the advantage of the process of the invention, whichallows, by means of coating the carbon nanotubes, deaggregation of the“bundles” that they have a natural tendency to form, and, as a result,relatively homogeneous dispersion of these nanotubes in a polyethylenematrix by melt blending. It may thus genuinely be considered that theaddition of the masterbatch to the HDPE matrix results in the formationof a nanocomposite within the strict meaning of the term.

In contrast, melt blending of the untreated carbon nanotubes and of HDPEdoes not lead to the production of a nanocomposite, but to theproduction of a microcomposite for which “bundles” of nanotubes arefound in the polyethylene.

Description of a Second Preferred Embodiment of the Invention

Polyethylene-based nanocomposites comprising double-walled carbonnanotubes (DWNTs) as fillers were also prepared according to the processof the present invention. The experimental conditions were as follows.

0.8 gram of DWNTs was dried overnight under reduced pressure at 105° C.A solution of MAO freed of TMA (46.5 ml of a solution with an aluminiumconcentration of 0.8 M) was added to the nanotubes. After removal of thesolvents in order to promote the binding of the MAO to the carbonnanotubes, the mixture was heated at 150° C. for 90 minutes. Thenon-bound MAO was removed by washing with dry toluene and was titratedto determine the amount of bound MAO (24.6 mmol bound). 200 ml ofn-heptane and 18.4 μmol of Cp*₂ZrCl₂ were added to the MAO-treatedDWNTs. The polymerization was performed at a pressure of 1.1 bar ofethylene and at 50° C. Various sample fractions of composites(nanocomposites) were taken during the synthesis of the polyethylene andprecipitated from acidified methanol for analysis. Prior to theanalysis, the nanocomposites, with an increasing content ofpolyethylene, were dried at 150° C. for 90 minutes under vacuum.

A DSC analysis in closed capsules was then performed and demonstratedthat the melting point and the degree of crystallinity increased as thepolyethylene content in the nanocomposite increased (data not shown).

An analysis of the nanocomposites by TGA was also performed andconfirmed the results obtained by DSC and that nanocomposites with anincreasing content of polyethylene were indeed obtained when thesampling time period increased (data not shown).

Description of a Third Preferred Embodiment

In a third preferred embodiment, polyethylene-based nanocompositescomprising single-walled carbon nanotubes (SWNTs) as fillers wereprepared according to the process of the present invention. Theexperimental conditions were the same as those described in the firstembodiment for nanotubes of MWNT type.

A comparison of the results obtained for the composite thus producedaccording to the process of the invention with those obtained for apolymer obtained by simple polymerization of ethylene is presented inTable 7.

As illustrated in this Table 7, compared with a simple polymerization inthe absence of nanotubes, the process of the present invention usingcarbon nanotubes of SWNT type as fillers makes it possible to increasethe catalytic activity of the polymerization.

In addition, a comparative study of the polymerization kinetics for asimple polymerization and for polymerization in the presence ofnanotubes of SWNT type according to the process of the invention asdescribed above demonstrated that these kinetics were comparable duringthe first twenty minutes, but that thereafter the rate of the simplepolymerization begins to attenuate whereas the rate of thepolymerization according to the invention continues to increase (seeFIG. 11).

A thermal analysis of two samples obtained by simple polymerization(Dabo B 013 a) and by polymerization on SWNT nanotubes according to theinvention (composite Dabo B 012) was also performed, and the results arepresented in Table 9.

It emerges from this analysis that the polymer alone obtained by simplepolymerization has a lower melting point than that of the compositeobtained according to the invention, but that its degree ofcrystallinity is higher.

A thermal study of polyethylene-based nanocomposites comprising carbonnanotubes of SWNT type and obtained according to the invention with anincreasing polyethylene content was performed, the results of which,presented in Table 8, made it possible to demonstrate that the productsobtained for an increasing sampling time period did indeed correspond tocomposites with an increasing polyethylene content and that for anincreasing amount of polyethylene formed, an increase in the meltingpoint and an increase in the degree of crystallinity could be observed.

Description of a Fourth Preferred Embodiment

A composite material was prepared according to the process of theinvention described in detail in the first embodiment, but with a fewdifferences. A minimum amount of 1.23 mmol of MAO was used, sinceprevious experiments had demonstrated that there was alwayspolymerization of the ethylene on the carbon nanotubes with this amountof MAO. In addition, the step for removal of the TMA and the steps ofwashing of the carbon nanotubes after heating them to 150° C. wereeliminated. Furthermore, 1 g of MWNTs was treated with MAO containing4.9 mmol of aluminium and using 40 ml of n-heptane.

For the homopolymerization of ethylene, 175 ml of n-heptane and 16.4μmol of Cp*₂ZrCl₂ were used.

With the aim of using it as a “masterbatch”, the composite thus obtainedwas subjected to thermogravimetric analyses. The analyses revealed thatthe composition of the composite was as follows:

-   -   39.7% by weight of polyethylene (obtained by TGA in helium 20°        C./minute)    -   5.8% by weight of alumina (obtained by TGA in air 20° C./minute)    -   54.5% by weight of MWNTs (determined by subtraction given the        amount of polyethylene and of alumina).

Verification was made that polyethylene (PE) had indeed been synthesized(by differential scanning calorimetry analysis (DSC)): thecharacteristic melting point of the PE measured was 134.4° C.(determined during the second passage in cyclic DSC (10° C./minute)) andthe degree of crystallinity was 51% (calculated from the heat of fusionduring the second passage in cyclic DSC).

This masterbatch thus obtained was redispersed by extrusion/injection ina high-density matrix (HDPE) of commercial type (from the company Dow,of MI₂=1.1 g/10 minutes) so as to obtain a new composite. Theexperimental conditions were as follows:

-   -   operating temperature: 190° C.    -   admission period: 4 minutes at 30 rpm    -   recirculation period: 6 minutes at 60 rpm    -   recovery: 2 minutes at 60 rpm    -   injection chamber temperature: 190° C.    -   injection temperature (mould): 100° C. (each process run results        in the production of two tensile test samples).

In parallel, samples containing either polyethylene alone or compositesobtained by simple direct mixing and comprising polyethylene and 1% byweight of multi-walled carbon nanotubes (MWNTs) were prepared.

Furthermore, an MWNT-based “masterbatch” was synthesized and itscomposition was also determined as being 39.7% by weight ofpolyethylene; 5.8% by weight of alumina and 54.5% by weight of MWNTs.

The three types of sample were then subjected to tensile tests with atensile speed of 50 mm/minute, and the results are presented below.

Breaking Stress and Elongation at Break:

As illustrated by FIGS. 12 and 13, the ultimate properties (breakingstress and elongation at break) are much better for the compositeobtained by redispersion of the masterbatch according to the inventionthan for polyethylene alone and the composite obtained by direct mixing.

Young's Modulus:

FIG. 14 shows that the Young's modulus values for the various materialsobtained during the injection are within the same range of values andthat these values are located within the relative measurement errors foreach of the materials.

Stress and Elongation at the Yield Point:

The same observations were made for the values obtained for the yieldpoint stress (FIG. 15) and the elongation at the yield point (FIG. 16).

In conclusion, the results obtained demonstrate that the use of acomposite based on a polymer matrix and comprising as filler carbonnanotubes obtained according to the invention and used as masterbatchmakes it possible to increase the ultimate properties especiallycompared with a polymer alone, but also compared with a compositeobtained by direct mixing of said polymer and of said filler. In otherwords, the process according to the invention makes it possible toobtain nanocomposites in which there is a transfer of the ultimateproperties of the carbon nanotubes to the composite, as desired.

Additional tests should make it possible to demonstrate that thistransfer of the properties of the carbon nanotubes to the composite alsoconcerns other physical properties.

Tables

TABLE 1 Mass of Total Max. weight Degree of crude mass of PEcrystallinity Cp*₂ZrCl₂ MAO MWNTs obtained obtained m.p. of the PESample (μmol) (mmol) used (g) (g) (g) (° C.) (weight %) 24 a 0 11.5 0.10.240 0.140 128.3 18 24 b 11.5 0 0.1 0.100 0 / / 28 a 0 0 0.1 0.100 0 // 28 b 0 11.5 0 0.038 0.038 131.0 25

Results obtained during the various ethylene polymerization testsstarting with crude MWNTs (2.7 bars of ethylene, 50° C., 30 minutes)TABLE 2 Al/Zr m_(crude MWNTs) m_(composites) m_(PE) Activity Crude MWNTsobtained Sample (mol/mol) (g) (g) (g) (kg/mol_(Zr) · h) by weight(weight %) 16 1000 0.25 10.21 9.96 866 2.5 11 1000 0 / 9.90 861 0

TABLE 3 Al/Zr m_(crude MWNTs) m_(composites) m_(PE) Activity Crude MWNTsobtained Sample (mol/mol) (g) (g) (g) (kg/mol_(Zr) · h) by weight(weight %) 21¹ 450 0.25 14.37 14.12 1228 2.5 23¹ 450 0 / 9.79 851 /N.b.: ¹11.5 μmol Cp*ZrCl₂; V_(heptane) total: 100 ml 1 h, 50° C. at 2.7bars of ethylene (0.25 g of MWNTs)

TABLE 4 Volume Weight W_(c) PE PE Sampling taken obtained m.p.(alone)^(b,c) content^(d) time^(a) (ml) (g) (° C.)^(c) (%) (weight %) t₁(Dabo 30 a) 38 0.388 130.9 45 42 t₂ (Dabo 30 b) 40 0.637 132.1 60 57.5t₃ (Dabo 30 c) 39 0.769 132.8 68 71.2 t₄ (Dabo 30 d) 50 1.419 132.9 6673.7 t₅ (Dabo 30 e) 39 1.577 133.1 66 78.3N.b.: P_(ethylene) = 1.1 bar; T = 50° C.; 1 g of MWNTs/200 ml ofn-heptane; 46 μmol Zr/g MWNTs; Al/Zr = 240^(a)Time elapsed between each sample withdrawal: 1 to 2 minutes^(b)Degree of crystallinity calculated for the PE matrix aftersubtraction of the filler content determined by TGA^(c)Values obtained during the second passage in DSC Values obtained byTGA in helium

TABLE 5 Breaking Young's Yield point MFI Breaking strain modulus stressYield point (g/10 Mixtures stress (MPa) (MPa) (MPa) (MPa) strain (%)min) Dabo   31 ± 1.6 744 ± 41 386 ± 44 24.4 ± 0.3   10 ± 0.5 1.01 40aDabo 15.3 ± 1.1  98 ± 20 419 ± 46 25.2 ± 2.2 11.2 ± 2.5 0.53 40b Dabo21.7 ± 2.4 468 ± 55 414 ± 30 25.5 ± 0.5 10 ± 1 0.70 40c

TABLE 6 m.p. W_(c) PE T_(deg.) in T_(deg.) in Mixture (° C.) alone (%)air (° C.)^(a) He (° C.)^(a) Dabo 40a 134.5 62.6 421 493 Dabo 40b 134.962.4 473 498 Dabo 40c 135.6 61.7 485 498^(a)Determined by the deriv. maximum of the thermogram (see FIG. 6).

TABLE 7 Amount of “bound” Mass PE Activity Sample SWNTs MAO [Al]/[Zr]obtained (kg/mol_(Zr) · h) Dabo B No 9 mmol 783 15.64 g 1360 013a Dabo BYes 9 mmol 783 21.28 g 1850 012

TABLE 8 Sampling Wt % of Wt % of Wt % of Wt % of m.p. W_(c) PE time^(a)Al₂O₃ ^(b) PE^(c) H₂O^(c) SWNTs^(d) (° C.)^(e) (alone)^(f) t₁ (Dabo B015a) 25.5 30.2 16.6 27.7 130.0 26 t₂ (Dabo B 015b) 20.6 48.4 13.7 17.3131.5 43 t₃ (Dabo B 015c) 15.3 63.4 10.2 11.1 133.5 51 t₄ (Dabo B 015d)5.1 82.8 4.1 7.2 133.8 57^(a)Time elapsed between each sample withdrawal is about 1 to 2 minutes;^(b)Determined by TGA in air (20° C./min, taken at 900° C.);^(c)Determined by TGA in helium (20° C./min);^(d)Determined by virtue of knowledge of the amount of PE, alumina andwater;^(e)Determined during the second passage in cyclic DSC (10° C./min) onsamples dried at 150° C.;^(f)Degree of crystallinity calculated on the basis of the PE obtainedby TGA on samples dried at 150° C.

TABLE 9 m.p. Wc Alumina content^(c) NT content^(d) Sample SWNTs (°C.)^(a) (%)^(b) (weight %) (weight %) Dabo B 013a No 132.9 71 n.d. n.d.Dabo B 012 Yes 135.3 59 1.2 2.2^(a)Determined during the second passage in cyclic DSC (10° C./min);^(b)Calculated from the heat of fusion during the second passage incyclic DSC;^(c)Determined by TGA in air (20° C./min);^(d)Determined by TGA in helium (20° C./min) from which is subtractedthe reside of the TGA in air.

REFERENCES

-   [1] A. B. Morgan, J. W. Gilman, T. Kashiwagi, C. L. Jackson;    Flammability of polymer-clay nanocomposites (Mar. 12-15 2000), the    National Institute of Standards and Technology.-   [2] F. Gao; e-Polymers (2002), No. T-004.-   [3] P. M. Ajayan; Chem. Rev. (1999) 99, 1787-1799.-   [4] B. G. Demczyk, Y. M. Wang, J. Cumings, M. Hetman, W. Han, A.    Zettl, R. O. Ritchie; Mater. Sci. Eng. (2002) A334, 173-178.-   [5] Kin-Tak Lau, D. Hui; Composites Part B: Eng., (2002) 33,    263-277.-   [6] E. T. Thostenson, Z. Ren, Tsu-Wie Chou; Composite Sci.    Tech. (2001) 61, 1899-1912.-   [7] H. Hagimoto, T. Shiono, T. Ikeda; Macromolecules (2002) 35,    5744-5745.-   [8] Dubois P. et al.; J. Macromol. Sci., Rev. Macromol. Chem.    Phys. (1998) C38, 511-566.

1. A process for obtaining a composite material comprising at least onepolymer matrix obtained by polymerization of a monomer referred to as a“monomer of interest” into a polymer, referred to as a “polymer ofinterest”, in the presence of carbon nanotubes homogeneously dispersedin said polymer matrix, said process comprising: using said carbonnanotubes as catalysis support to bind homogeneously at the surfacethereof a cocatalyst/catalyst couple so as to form a catalytic system;activating said catalytic system for polymerization; polymerizing saidmonomer at the surface of the carbon nanotubes using said activecatalytic system, the polymerization being allowed to progress over timeso as thus to obtain said polymer matrix around said carbon nanotubes,as the polymerization of said monomer proceeds.
 2. The process accordingto claim 1, further comprising the following steps: preparing asuspension of carbon nanotubes in an inert solvent; pretreating saidcarbon nanotubes by adding said cocatalyst, so as to obtain a suspensionof pretreated carbon nanotubes in which the cocatalyst is adsorbed ontothe surface of the carbon nanotubes; preparing a reaction mixture fromthe suspension of carbon nanotubes thus pretreated, by adding thecatalyst and circulating a flow of monomer in said suspension ofpretreated nanotubes, so as to bring about in said reaction mixture thepolymerization of said monomer at the surface of said nanotubes and thusto form the composite material, in which said carbon nanotubes arecoated with said polymer of interest; stopping the polymerizationreaction when the polymerization in the reaction mixture has reached arate of polymerization of between about 0.1% and about 99.9%.
 3. Theprocess according to claim 1, wherein said monomer is an olefin and saidpolymer of interest is a polyolefin.
 4. The process according to claim1, wherein said monomer of interest is selected from the groupconsisting of ethylene, propylene, copolymers thereof withalpha-olefins, conjugated alpha-diolefins, styrene, cycloalkenes,norbornene, norbornadiene, cyclopentadiene, and mixtures thereof.
 5. Theprocess according to claim 3, wherein said polymer of interest ispolyethylene.
 6. The process according to claim 1, wherein thecocatalyst/catalyst couple and the experimental parameters are chosen insuch a way that the catalyst can be immobilized at the surface of thecarbon nanotubes by means of the cocatalyst in order to thus form thecatalytic system.
 7. The process according to claim 1, wherein thecatalyst is capable of catalysing the polymerization of the monomer ofinterest and is selected from the group consisting of metallocenes,hindered amidoaryl chelates, hindered oxoaryl chelates, Fe (II and III)and Co (II) bis(imino)pyridines, and Brookhart complexes based on Ni(II), Pd (II), and mixtures thereof.
 8. The process according to claim1, wherein the cocatalyst is methylaluminoxane or a chemically modifiedmethylaluminoxane, or a mixture thereof.
 9. The process according toclaim 1, wherein the cocatalyst/catalyst catalytic couple is themethylaluminoxane/Cp*₂ZrCl₂ couple.
 10. The process according to claim1, wherein the amount of catalyst is between about 10⁻⁶ and about 10⁻⁵mol/g of carbon nanotubes.
 11. The process according to claim 1, whereinthe amount of cocatalyst in the reaction mixture is between about 10-3and about 10-2 mol/g of carbon nanotubes.
 12. The process according toclaim 2, wherein the temperature of the reaction mixture is between 25°and 140° C.
 13. The process according to claim 2, wherein thepretreatment is performed at a temperature of between 25° C. and 200° C.for a time period of between 1 minute and 2 hours.
 14. The processaccording to claim 1, wherein the polymerization is performed at apressure of between about 1 and about 3 bars of monomer.
 15. The processaccording to claim 1, wherein the polymerization is performed at apressure of between about 1.1 and about 2.7 bars of monomer.
 16. Theprocess according to claim 2, wherein, in order to prepare the reactionmixture, the catalyst is added to the suspension of pretreated carbonnanotubes before circulating the flow of monomer in said suspension. 17.The process according to claim 2, wherein, in order to prepare thereaction mixture, the addition of the catalyst to the suspension ofpretreated carbon nanotubes and the circulation of the flow of monomerin said suspension are concomitant.
 18. The process according to claim1, wherein the carbon nanotubes are selected from the group consistingof single-walled carbon nanotubes, double-walled carbon nanotubes andmulti-walled carbon nanotubes, and/or mixtures thereof.
 19. The processaccording to claim 1, wherein the carbon nanotubes are crude and/orpurified carbon nanotubes.
 20. The process according to claim 1, whereinthe carbon nanotubes are functionalized carbon nanotubes.
 21. Theprocess according to claim 2, wherein the polymerization reaction isstopped when the rate of polymerization is such that the compositecomprises between about 50% and about 99.9% of carbon nanotubes andbetween about 50% and 0.1% of polymer.
 22. The process according toclaim 2, wherein the polymerization reaction is stopped when thenanocomposite formed comprises between about 0.1% and about 50% ofcarbon nanotubes homogeneously dispersed at the nanoscopic scale in thepolymer matrix, and between about 99.9% and 50% of polymer.
 23. Theprocess according to claim 1, further comprising an additional stepduring which the composite material is used as a masterbatch to preparea nanocomposite based on a polymer known as an “addition polymer”, saidaddition polymer being miscible and compatible with the polymer ofinterest of the composite material.
 24. A catalytic system forperforming the process according to claim 1, consisting of carbonnanotubes, a cocatalyst and a catalyst, said catalyst forming with saidcocatalyst a catalytic couple, in which said catalyst and saidcocatalyst are bound to the surface of said carbon nanotubes.
 25. Acomposition for performing the process according to claim 1 andcomprising a catalytic system, the catalyst being selected from thegroup consisting of metallocenes, hindered amidoaryl chelates, hinderedoxoaryl chelates, Fe (II and III) and Co (II) bis(imino)pyridines,Brookhart complexes based on Ni (II) and Pd(II), and mixtures thereof,and the cocatalyst being methylaluminoxane or a chemically modifiedmethylaluminoxane, or a mixture thereof.
 26. A composite materialobtained by the process according to claim
 1. 27. The composite materialaccording to claim 26, comprising between about 0.1% and 99.9% of carbonnanotubes and between about 99.9% and 0.1% of polymer.
 28. The compositematerial obtained by the process according to claim 1 and correspondingto a nanocomposite comprising at least one matrix of at least onepolymer, in which carbon nanotubes are homogeneously dispersed at thenanoscopic scale.
 29. The composite material according to claim 28,comprising between about 0.1% and about 50% of carbon nanotubes andbetween about 99.9% and about 50% of polymer.
 30. The composite materialaccording to claim 26, wherein the carbon nanotubes are coated withpolymer.
 31. A composite material comprising a matrix of at least oneaddition polymer and the composite material according to claim
 26. 32.(canceled)
 33. A process for polymerizing a monomer, comprising usingthe process according to claim 1, the polymerization reaction beingallowed to proceed for a period sufficiently long so as to have acontent of carbon nanotubes of less than 0.1% and a polymer content ofgreater than 99.9%.
 34. A polymer obtained by the process according toclaim
 33. 35. A catalytic system for performing the process according toclaim 23, consisting of carbon nanotubes, a cocatalyst and a catalyst,said catalyst forming with said cocatalyst a catalytic couple, in whichsaid catalyst and said cocatalyst are bound to the surface of saidcarbon nanotubes.
 36. A composition for performing the process accordingto claim 23 and comprising the catalytic system according to claim 35,the catalyst being selected from the group consisting of metallocenes,hindered amidoaryl chelates, hindered oxoaryl chelates, Fe (II and III)and Co (II) bis(imino)pyridines, Brookhart complexes based on Ni (II)and Pd (II), and mixtures thereof, and the cocatalyst beingmethylaluminoxane or a chemically modified methylaluminoxane, or amixture thereof.
 37. A composite material obtained by the processaccording to claim
 23. 38. The composite material according to claim 37,comprising between about 0.1% and 99.9% of carbon nanotubes and betweenabout 99.9% and 0.1% of polymer.
 39. The composite material obtained bythe process according to claim 23 and corresponding to a nanocompositecomprising at least one matrix of at least one polymer, in which carbonnanotubes are homogeneously dispersed at the nanoscopic scale.
 40. Thecomposite material according to claim 39, comprising between about 0.1%and about 50% of carbon nanotubes and between about 99.9% and about 50%of polymer.
 41. The composite material according to claim 37, whereinthe carbon nanotubes are coated with polymer.
 42. A composite materialcomprising a matrix of at least one addition polymer and the compositematerial according to claim
 37. 44. A process for polymerizing amonomer, comprising using the process according to claim 23, thepolymerization reaction being allowed to proceed for a periodsufficiently long so as to have a content of carbon nanotubes of lessthan 0.1% and a polymer content of greater than 99.9%.
 45. A polymerobtained by the process according to claim 44.